1. Field of the Invention
The present invention relates to chopper control apparatus.
2. Description of the Related Art
A chopper is a device fed with a DC input magnitude (voltage or current), and including at least one power-switch component that is opened or closed so as to set the mean value for an output magnitude (current or voltage) of the device, the on duration of said switch component being defined by the width of control square-waves which are applied to it, and the rate of the square-waves setting the operating rate of the chopper.
To control a chopper circuit, the following steps are performed:
measuring the mean value of the output magnitude of the chopper; and PA1 regulation calculation for determining the width of the control square-waves to be applied to said switch, so as to make said mean value equal to a determined reference magnitude. PA1 means for generating a sampling signal having pulses whose positions, relative to said control square-waves, are determined so that said pulses coincide with said output magnitude passing through its mean value; and PA1 sampling means for sampling said output magnitude by means of said sampling signal.
In general, these mean value measurement and regulation calculation steps are performed at the operating frequency of the chopper, and, in general, said control square-waves are further generated by setting one of the control instants (opening or closing) of said switch, the other instant being controlled as a function of the results of said regulation calculation.
A problem arises when measuring the mean value of the output magnitude of the chopper: the harmonics due to the chopping must be removed from the output magnitude. In general, this problem is solved by performing filtering, which may be digital or analog.
A major drawback of such filtering is that it results in a relatively long time being required for acquiring the mean value (which time, added to the regulation calculation time, means that the result of the calculation is, in general, applied one period late). Another major drawback of such filtering is that, depending on whether the filtering is analog or digital, it is complex to implement or it gives rise to a limitation in the passband (because of the pre-filtering step that is necessary to avoid the phenomenon of spectrum folding by sampling of the output magnitude, which phenomenon is due to the digital processing the output magnitude).